IPoIB: Fix usage of uninitialized multicast objects

The driver should avoid calling ib_sa_free_multicast on the mcast->mc
object until it finishes its initialization state.  Otherwise we can
crash when ipoib_mcast_dev_flush() attempts to use the uninitialized
multicast object.

Instead, only call wait_for_completion() for multicast entries that
started the join process, meaning that ib_sa_join_multicast() finished.

Signed-off-by: Erez Shitrit <erezsh@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/ulp/ipoib/ipoib.h

@@ -101,6 +101,7 @@ enum {
 	IPOIB_MCAST_FLAG_SENDONLY = 1,
 	IPOIB_MCAST_FLAG_BUSY	  = 2,	/* joining or already joined */
 	IPOIB_MCAST_FLAG_ATTACHED = 3,
+	IPOIB_MCAST_JOIN_STARTED  = 4,
 
 	MAX_SEND_CQE		  = 16,
 	IPOIB_CM_COPYBREAK	  = 256,
@@ -151,6 +152,7 @@ struct ipoib_mcast {
 	struct sk_buff_head pkt_queue;
 
 	struct net_device *dev;
+	struct completion done;
 };
 
 struct ipoib_rx_buf {